The recovery of small instruments moored in coastal waters has been hampered by uncontrolled factors, due primarily to the marker buoy at the surface. Many instruments are lost because the float becomes entangled in fishermen's gear, or, because the float attracts unwanted attention. Commercially available transponder releases have been used on many moorings which generally operate at greater depths. Most of these devices are heavy, bulky, complex devices, which require preparation by skilled technicians, and not suitable for deployment from the small open boats generally used for near short work. Most of the present devices are also expensive relative to the value of some of the instruments themselves.
U.S. Pat. No. 3,858,166 discloses a device that employs an acoustic signal to trigger a magnetic latch for a buoyant marker. The magnetic latch releases the marker when supplied with a current that neutralizes the magnetic field of the permanent magnetic. To effect release in this manner requires a substantial current for a substantial period of time and hence has a high power requirement relative tot he strength of the permanent magnet. To allow use of a relatively small strength magnet, a lever-arm mechanism is employed.
Electrically promoted magnetic flux diversion is a known concept that has been employed in cranes used to move scrap metal. The known device uses a permanent magnet in combination with a control winding that generates a magnetic field with polarity opposite to that of the permanent magnet, when supplied with a electric current, for releasing a load.
U.S. Pat. No. 4,664,559 discloses a remotely operated magnetic release for anchored aquatic instrumentation which includes a geared mechanism for effecting release of the buoyed instrument for recovery.
For the recovery of small instruments moored in coastal waters it is desirable to have a simple, low cost, low powered, maintenance free device without moving parts, that would allow a small tethered buoy or float to rise to the surface when recovery is desired.